In conventional wireless communication systems, such as wideband code division multiple access (WCDMA) Release 5/6, high speed data transmission is achieved by high speed downlink packet access (HSDPA) and high speed uplink packet access (HSUPA) technologies. To improve reliability of data transmission, H-ARQ and ARQ are implemented.
FIG. 1 shows a conventional wireless communication system 100. The system 100 includes a user equipment (UE) 110, a Node-B 120 and a radio network controller (RNC) 130. H-ARQ entities are located in a medium access control (MAC) layer 112 of the UE 110 and a MAC layer 122 of the Node-B 120. Packets are assigned transmission sequence numbers (TSNs) in an H-ARQ transmitter. An H-ARQ receiver receives the packets and attempts to decode the transmitted packets. If a packet is not decodable, the H-ARQ receiver sends a negative acknowledgment (NACK) back to the H-ARQ transmitter for retransmission of the failed packet. If a packet is correctly decoded, the H-ARQ receiver sends a positive acknowledgment (ACK) for the packet to the H-ARQ transmitter. Upon receiving a NACK, the H-ARQ transmitter retransmits the packet if the number of retransmissions of the failed packet is less than a predetermined maximum limit, and the allowed transmission time for the failed packet has not expired. Otherwise, the failed packet is discarded and recovered at an ARQ level.
ARQ entities are located in a radio link control (RLC) layer 114 of the UE 110 and an RLC layer 132 of the RNC 130. The ARQ entities handle the retransmission of the failed packet. The ARQ entities recover packets that are lost due to an NACK-to-ACK misinterpretation error at the H-ARQ level. The ARQ process in an RLC layer is quite complex with many options to perform status reporting.
An H-ARQ assisted ARQ operation may be considered when both the H-ARQ and ARQ functions are co-located, such as in a universal mobile telecommunication systems (UMTS) Node-B or a long term evolution (LTE) evolved Node-B (eNode-B).
FIG. 2 shows an example of an H-ARQ-assisted ARQ operation proposed for third generation partnership project (3GPP) standards. A transmitter 250 includes an ARQ transmitter 252 and an H-ARQ transmitter 254. A receiver 260 includes an ARQ receiver 262 and an H-ARQ receiver 264. The H-ARQ transmitter 254 provides a local ACK or a local NACK to the ARQ transmitter 252.
As shown in FIG. 2, a local NACK is generated when the H-ARQ transmitter 254 fails the H-ARQ transmission, (e.g., due to maximum retransmission limit). The ARQ transmitter 252 sends an ARQ protocol data unit (PDU) x to the H-ARQ transmitter 254 (step 202). The H-ARQ transmitter 254 sends the ARQ PDU x to the H-ARQ receiver 264 (step 204). The ARQ PDU x is not decodable and the H-ARQ receiver 264 sends a NACK to the H-ARQ transmitter 254 (step 206). The H-ARQ transmitter 254 retransmits the ARQ PDU x to the H-ARQ receiver 264 (step 208). The ARQ PDU x is still not decodable and the H-ARQ receiver 264 sends another NACK to the H-ARQ transmitter 254 (step 210). At such a point, it is determined that the number of retransmissions for the ARQ PDU x reaches a maximum retransmission limit (step 212). The H-ARQ transmitter 254 then sends a local NACK for the ARQ PDU x to the ARQ transmitter 252 (step 214).
A local NACK may also be generated when an NACK-to-ACK error is reported from the H-ARQ receiver 264 to the H-ARQ transmitter 254. Still referring to FIG. 2, the ARQ transmitter 252 sends an ARQ PDU y to the H-ARQ transmitter 254 (step 216). The H-ARQ transmitter 254 transmits the ARQ PDU y to the H-ARQ receiver 264 (step 218). The ARQ PDU y is not decodable and the H-ARQ receiver 264 sends a NACK to the H-ARQ transmitter 254 (step 220). However, the NACK is misinterpreted as an ACK by the H-ARQ transmitter 254 and the H-ARQ transmitter 254 treats the ARQ PCU y as successfully transmitted. The H-ARQ receiver 264 detects an NACK-to-ACK error, (e.g., when the H-ARQ receiver 264 receives a new PDU via the same H-ARQ process while waiting for retransmission of the ARQ PDU y), (step 222). The H-ARQ receiver 264 sends a NACK-to-ACK error indicator to the H-ARQ transmitter 254 (step 224). Upon receipt of the NACK-to-ACK error indicator, the H-ARQ transmitter 254 sends a local NACK to the ARQ transmitter 252 and the ARQ PDU y is recovered at an ARQ level (step 226).
As shown in FIG. 2, a local ACK is generated when none of the above two events for an ARQ packet occurs during a predefined time interval. The ARQ transmitter 252 sends an ARQ PDU z to the H-ARQ transmitter 254 (step 228). The H-ARQ transmitter 254 transmits the ARQ PDU z to the H-ARQ receiver 364 (step 230). The ARQ PDU z is successfully decoded and the H-ARQ receiver 264 sends the ARQ PDU z to the ARQ receiver 262 (step 232), and sends an ACK to the H-ARQ transmitter 254 (step 234). When it is determined that a NACK-to-ACK error is not reported during a predetermined time interval (step 236), the H-ARQ transmitter 254 sends a local ACK to the ARQ transmitter 252 (step 238). The ARQ transmitter 252 will discard the packet from a transmit buffer after receiving the local ACK from the H-ARQ transmitter 254.
In the above example of an H-ARQ assisted ARQ system, a complex layer 2 (L2) status reporting mechanism is removed under the assumption that the NACK-to-ACK error is the only significant source of undetected packet loss. The ARQ transmitter will not be able to recover the packet for cases where the NACK-to-ACK error indicator is lost or a shared channel is lost due to a discontinuous transmission (DTX)/ACK error. Thus lossless transmission cannot be guaranteed.
Current downlink H-ARQ, (Release 5), does not specify the maximum number of retransmissions for a packet. Thus, the H-ARQ transmitter or receiver does not know the maximum number of transmissions for each packet. There is an assumption that the ARQ transmitter and the H-ARQ transmitter have the same PDU size. The H-ARQ transmitter informs the ARQ transmitter the segment that is lost, (not the ARQ PDU). If the ARQ PDU is retransmitted, the H-ARQ transmitter will retransmit the complete packet, not only the missing segment. It is also unclear how to recover the last packet of the H-ARQ transmission.